To chart comparison values or combine two related indicators on the same chart, you may need to compensate for different numeric scales.
For example, you might like to chart the sales of your two fastest growing product lines together to get an idea of how business is growing. However, while the growth rates are similar, the two product lines may sell at entirely different volumes. If this is the case, the chart won't provide much comparative information because each line needs to be charted at a different scale.
By charting each item to a different axis, you can include both items correctly in the same chart. Each item in the values panes can be assigned to one of two axes, each of which can be auto- or manually scaled.
To add a second Y-axis:
Double-click an item in the values panes.
A second Y-axis is added to the right of the chart, and is auto-scaled to match the item. The item is displayed in italics in the data layout.
If desired, change the scale or modify the second axis using the Chart Values Axis Properties tab of the Chart Properties dialog box.